1. Field of the Invention
The present invention relates to the heating of structural materials and more particularly to a structural composite which is electrically conductive and can be electrically heated.
2. Description of the Related Art
Ice forms on the forward-facing surfaces of an aircraft when it flies through clouds of super-cooled water droplets, i.e. droplets having temperatures less than 32.degree. F. These droplets, which are very small (typically 20 microns in diameter, compared to 1,000 microns for a rain droplet) freeze almost instantaneously upon impact with the aircraft, resulting in the formation of ice. When it is determined that ice build up is unacceptable, then measurements must be taken to assure that no ice build up occurs or is at least kept within acceptable limits. There are essentially two means for providing "active" ice protection (ice protection provided by systems carried on board the aircraft as opposed to passive means, such as accelerating the aircraft to such a speed that aerodynamic heating maintains the ice accreting surfaces above freezing)--de-icing and anti-icing.
In de-icing, ice is allowed to build up on the protected surfaces then periodically shed before its size/thickness becomes unacceptable. Typical systems used for de-icing include electro-thermal systems and mechanical systems. Electro-thermal systems utilize a component's electrical resistance heating. Examples include wires, etched foil or spray coatings. Mechanical systems include pneumatic "boots".
For anti-icing, ice may or may not be allowed to initially accrete on the protected surface; however, once the ice is shed the surface is maintained ice-free by the anti-icing system for the remainder of the ice encounter. Electro-thermal systems using electrical resistance heating are used for de-icing in a similar manner as they are used for anti-icing; but, the electrical power is continuously applied so as to maintain surface temperatures above freezing. Other typically used anti-icing systems include hot gas systems and chemical fluid systems. Hot gas systems are usually designed directly into the structure of the aircraft and utilize hot air bleed from the engine as a source of heat. Chemical fluid anti-icing is accomplished by dispensing a fluid onto the surface to be iced, through small holes in the surface. These fluids depress the freezing temperature of water on the surface well below 32.degree. F. so that the impinging water droplets do not form ice.
De-icing and anti-icing problems associated with airplanes include the wing, upon which unsymmetrical ice shedding may cause roll problems. Furthermore, wing leading edge ice causes drag increase, loss of lift and increased stall speed. Ice on the leading edge of the engine inlet may damage the engine while shedding. The formation of ice must be controlled on the leading edges of the horizontal and vertical stabilizers in order to prevent tail flutter. Ice formation must be controlled on the ailerons and flaps. Ice formation must also be controlled on the pitot tube drain and balance horns.
Disadvantages of thermal hot gas systems include the fact that they require high energy. Furthermore, future and some current engines cannot provide adequate bleed air. Chemical fluid systems are deficient in that they are time limited, have high weight and are expensive. Mechanical pneumatic boots suffer from rain/sand erosion. Furthermore, they require high maintenance, have limited life, result in increased drag and may collect residual ice. Current electro-thermal systems require high energy, are heavy and are expensive. Specifically, composite structural materials which utilize embedded electric wires through which current is passed to heat the composite structures are deficient inasmuch as the wires usually have a deleterious effect on the mechanical properties of the structure. The use of electrically conductive composites on aircraft has previously been described in the literature. However, such uses have not been for the purpose of de-icing or anti-icing. U.S. Pat. No. 4,599,255 entitled "Composite Structures Having Conductive Surfaces", issued to J. M. Anglin et al, discloses a conductive composite structural member for use in an aircraft structure for providing P-static conductivity, lightning strike protection, electrical shielding capability, and conductivity for antenna ground plane useage.